Is this a cassette? or a freewheel? What Brand? Do you have STI now or are you wanting to put STI on the bike? Do you have the parts laying around or were you going to buy them?
Chances are good it isn't going to work with shimano STI levers but it depends on your cassette/freewheel.
If you have a freewheel then I would recommend skipping this upgrade and just building with a new drivetrain to 9/10s with new wheels. An 8s freewheel wasn't long for this world when new and now are kinda like bigfoot. Also the indexes shifts wouldn't be real crisp.
If you have a cassette and the parts on hand then assuming the cassette and shifters are matched, then you can always just try it. It may or may not work and last time I checked shiftmate didn't make anything for Mavic, so if it doesn't work just plan on a new RD. Not all that expensive in the general scheme of things.
If you have to purchase all the parts then its another vote for just replacing the entire drive train, including hubs, for 9/10s.
1986 Alan Record Carbonio, 1985 Vitus Plus Carbone 7, 1984 Peugeot PSV, 1972 Line Seeker, 1986(est.) Medici Aerodynamic (Project), 1985(est.) Peugeot PY10FC
I don't think the Mavic 851 SSC RD was ever designed to work with indexed levers. It's the next, newer model (the 840) that did have the indexed feature. You can always experiment and see if the 851 will work with indexed levers, but it could be a long shot.
I know that the 851 does work best with Mavic/Simplex retrofriction (non-indexed) levers though, as I have them on two bikes set up as such at seven speeds. Not sure if it has enough movement range to handle another speed though, as it feels close to being stretched at seven already
1991 vintage, Italian Daccordi, with full Mavic gruppo.
I just discovered today (after twenty years of searching out this problem) that the 840's have *two* ways of running the cable: One, through a guidehole, for six speed cassettes; and another *around* that same guide, for seven and eight speeds. My 840, which indeed was indexed, never shifted properly, until I figured this out, as I could tweak it for one range or another, but not for both. So this one is perhaps not designed for index, as all that design was pretty specific.
I would think that non-index saves you a world of problems, not the least of which was above. Also, because of the indexing issue, they were designed for Mavic shifters AND rear cassette. As I remember, when Mavic began to stop producing parts, they offered a free (what's it called?) freebody, to allow the cassette to accommodate Shimano. So, again, some pretty detailed stuff to work with non-Mavic stuff. Non-indexing should make things much simpler.
that rear derailer definitely looks pre-indexing. note that the Mavid RD that Donzie is talking about is the standard "slant paralellogram" design, not the type that the OP is asking about.
To the OP: you'll need to do friction shifting, or just get a different rear derailer. Shimano index-compatible rear derailers are all over the place used, for cheap. There's no way (or VERY low likelihood) the derailer pictured will shift with a consistent throw across cogs.
I ran those 851 SSC derailleurs for years...they're light, easily rebuilt, simple and very durable. But they aren't meant for indexing. Like someone stated previously, you could try it, but it would be a longshot. If you want a really cool, retro set-up...find yourself a pair of old Kronos carbon friction shifters!